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slipped over the tube at the end of the furnace, may be used
with advantage.                                           _
The speed of the bubbles is the best indication of the progress
of the combustion. If the rate increases so that the bubblesi
passing through the last bulb cannot easily be counted, ubunilvr
or burners must be lowered or extinguished until the spcjtxi
slackens. After a time, when the air has been displaced and
carbon dioxide largely fills the tube, the ps is nearly all ul>^
sorbed in the first potash bull). When this occurs, the current
of oxygen maybe increased until the bubbles appear synchro-
nously in the bulbs, when the current is again checked. £ f
some copper oxide has been reduced in the first stages of thCi
process, the bubbles in the potash apparatus may entirely ceust*
for a time, but will reappear when the copper has been reox|_
dised. Here again an increased current of oxygen will hasten
the process. The combustion is complete when a glowing chip
held at the end of the potash apparatus is rekindled All the*
moisture must by now have been driven over into the calcium
chloride tube. If this is not the case, warm the end of the (t(1.>^*
cautiously with a small flame, or by means of a hot tile held
near the tube The time required to complete the combust ion
is about one-half to three-quarters of an hour from the time- tlic*
front of the tube is red hot, but more volatile substances, whtcli
must be heated more cautiously, will naturally take longer,
The combustion being complete, gradually turn down, and in
a few minutes extinguish, the burners. Whilst the furnace cooln
the oxygen is replaced by a slow current of air. To do this *!*<,*
oxygen supply is stopped and the three-way tap is turner I
through 180°, so as to connect the tube with the air rcsm*t>it%
the tap of which is then opened and the stream of air rt»gulatt*cl
by the screw clip.
Let the air pass through for 20 minutes whilst the furnace •&
cooling down. Then remove and stopper the potash apparatus.
and'the calcium chloride tube, and after allowing them to stand
by the balance case for half-an-hour, weigh.
The results are calculated in percentages of carbon and
hydrogen as follows :
w is the weight of substance taken,
a is the increase in weight of the potash apparatus.     *